This javascript cut & paste code snippet prevents the user from selecting a filename with a space (known to cause problems with some CGI-scripts). The visitors are informed that the field is not allowed to have spaces. You may change the character it checks for as well as the invalid alert message to something else to suit your needs.

Insert this code into your HEAD of HTML

<!-- // == This Script Free To Use Providing This Notice Remains == // // == This Script Has Been Found In The http://www.DesignerWiz.com Javascript Public Archive Library == // // == NOTICE: Though This Material May Have Been In A Public Depository, Certain Author Copyright Restrictions May Apply == // --><script language="JavaScript" type="text/javascript"> <!-- Begin function validate(){ var invalid = " "; // Invalid character is a space if (document.submitform.filename.value.indexOf(invalid) > -1) { alert("Sorry, spaces are not allowed."); return false; } else { return true; } } // End --> </script>

Insert this code into your BODY section

<center> <form name=submitform onSubmit="return validate()"> <input type=file name=filename> <p><input type="submit" value="Submit"></form> </center>

Simply Cut and Paste Script code
Be sure to properly place script position in HEAD & BODY sections of html 

This script has been in the DesignerWiz.com Public Library.